Yes, there is a foam/rubber gasket that seals out the elements and seals in the cold air. The "gasket" has edges that over lap the evaporator housing and the inside of the kick panel and finally the air duct assembly. This gasket is important to keep water out of the car and evaporator. You can see a picture of this in a 1966 chassi shop manuel on page 1A-65.

I know that I am writing you almost a year later, but I am restoring my AC now. I NEED THAT SEAL! Maybe you might have found a supplier/part number or figured a new way to seal that opening. Hope I help and I hope you can help me.